Integrating Counter Cell for Use with Vapor Phase Chromatography George Blyholder,' Department of Chemistry, The Johns Hopkins University, Baltimore, Md. OR
many experiments it is desirable
the area of the chromatographic peak, the quantity of ethane in the sample is determined. From the counting rate just after the ethane is condensed in the cell, the activity of this quantity of ethane is determined. The combination of these two facts gives the relative specific actirity of the ethane. All measured activities are relative to the same base, 17-hich depends on the counting efficiency of the cell. The ethane peak was followed by a peak for nonradioactive carbon dioxide. The carbon dioxide, n-hich is also condensed in the counter cell, does not appreciably decrease the counting rate. S e x t came a prak for ethTlene. The activity of the ethylene is given by the difference in the counting rates before and after the ethylene entered t h r counting cell. The counting rate appears as a series
F to combine radioactive tracer methods with chromatographic separation. The usual method is to lead the fractionated exit gas from the chromatographic apparatus directly into a counting cell. The gas to be counted flows directly through the Geiger tube itself or through a gas cell which is separated from the Geiger tube only by a n appropriate window. These methods m e sensitive and simple to set up but there is some difficulty in interpreting the specific activity of an individual component. If the nidth, measured in tinw units, of a chromatographic peak is less than the residence time of a slug of gas in the counter cell, the height of the> radioactive peak will be a measure of the total activity of the, fraction producing the chromatographic peak. If the peak nidth is considerably greater than the residence time in the countw cell, the a r m of the radioactive peak may be used as a measure of the activity of the fraction producing the chromatographic peak. I n the latter case and in a situation intermediate betn een the two cases described, if the specific activity of a component is desirrd, a calibration with that component possessing a known specific activity must be made. Moreover, if the conditions on the chromatographic column are changed so that the peak width changes, the old calibration is no longer valid. I n dealing with a number of radioactive products, this becomes a major task, to say nothing of the problem of obtaining a particular compound n-ith a known specific radioactivity. Rather than solve the above problems, a cell was designed in which the integrated activity of a slug of gas is determined regardless of the spread of the slug. The chromatographic separations were obtained using a Perkin-Elmer iModel 154 vapor phase fractometer. The essential feature of the cell (Figure I ) is that the component to be counted is condensed on a liquid nitrogen-coolrd surface just above the window of a Geiger tube. The gas inlet and exit are side by side, with a mica divider between them and extending two thirds across the cell. The gas from the fractometer enters the cell, circulates around the mica divider, and leaves the cell. All components except those VThich have an appreciable vapor pressure a t -195' C. are conPresent address, Department of Chemistry, University of Arkansas, Fayetteville, Ark.
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ANALYTICAL CHEMISTRY
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,+--GAS FROM FR A C T O M E T E R
I1 I /I
/I JL ELECTR G A L
QUENCHING
LFADS
G A S IN
GEIGER
Figure 1.
COUNTER
Schematic diagram of cell cop
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RADIOACTIVITY CHROMATOGRAM
Figure 2. Results on mixture of carbon- 14labeled gases
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DISTANCE
dcnsed on the cold surface. The mica window is cemented onto the flange a t the bottom of the window, above a Geiger tube. The activity of all components condensed in the cell is rrgis. tered on a counter rate meter. The results of running a mixture of carbon-14-labeled gases through the chromatographic and counting apparatus are shown in Figure 2. The carbon monoxide, which is nonradioactive, registers on the chromatogram but not on the counting rate plot. The radioactive methane registers on both plots, but because methane has an appreciable vapor pressure a t - 195" C., after the methane peak has passed the counting rate quickly drops to zero. The radioactive ethane is condensed in the counting cell. As long as there is liquid nitrogen in the cell, the ethane IT ill remain deposited on the cold surface and the counting rate Kill include the total activity of the ethane. From
FROM ORIGIN
of steps, each of which represents the integrated activity of a particular component separated by the chromatographic apparatus. The efficiency of condensation of gases in the cell \vas checked using ethane. An arrangement was made whereby any ethane not condensed in the counter cell could be detected in a chromatographic apparatus. S o ethane v a s detected !!-hen the counter cell mas cooled by liquid nitrogen. As a furthpr check on the performance of the counter c ~ l l a sample containing radioactive hydrocarbons from Cz to Cs mas analyzed by the technique described and by a static technique. Both methods yielded the same relative activitirs for all the hydrocarbons. Because of differences in the counter windoiy thicknesses, the measured activities were not the same. WORKsupported under a contract from the Atomic Energy Commission.
